When a flowable liquid composition is applied to a substrate to form a coating or decoration, or to provide graphic or other information, it may be advantageous shortly after application to obtain rapid hardening, gelling, or curing of the coated material by irradiation for a brief period of time. This is particularly advantageous if the liquid coating composition is substantially free of volatile solvents which do not themselves participate in the curing, since the hardening then may be effected very rapidly without interference from evolving vapors and without producing waste gases. Practical coating systems of these types have been developed, utilizing photosensitive latent curing catalyst which respond to irradiation by releasing the catalytic agent.
One such coating system is disclosed and claimed in my U.S. Pat. No. 3,708,296 issued Jan. 2, 1973 and utilizes epoxide compounds (or mixtures) of relatively low molecular weight, which may be formulated to provide good flow characteristics with or without the use of inert solvents. Cationic polymerization catalysts cause the epoxy ring to open through cleavage of a carbon-oxygen bond, probably forming a cationic reactive intermediate. The reaction thus initiated may repeat itself rapidly many times in a chain reaction to form a polymer of repeating ether units. Gelling time for such photosensitive catalytic polymerization may be short enough to provide a substantially hardened coating a short distance after irradiation is carried out while the substrate passes at high speed along a treatment line.
However, epoxide and related compositions containing photosensitive catalytic precursors have a tendency to gel upon standing, even in the absence of light or ultraviolet radiation. This tendency to undergo premature reaction is particularly troublesome in the case of formulations which are substantially free of unreactive diluents or solvents. The polymerization reaction is exothermal and, where large masses are involved, can generate sufficient heat to cause combustion of the epoxide resins. In U.S. Pat. No. 3,721,617 issued Mar. 20, 1973 to William R. Watt and commonly assigned herewith, certain cyclic amides, free of unsubstituted hydrogen on the amide nitrogen atom, for example polyvinylpyrrolidones, are disclosed and claimed as gellation inhibitors for polymerizable compositions comprising epoxides and mixtures of epoxides with other monomers. Such compositions are photosensitive and when exposed to an energy source such as actinic radiation yield epoxy polymers which are receptive to ink and possess inherent toughness, abrasion resistance, adherence to metal surfaces, etc., but prior to such exposure are stable compositions having extended storage or potlife, premature reaction in the dark or at minimal levels of radiation being inhibited so that the mixtures may be retained for periods of days or more before application.
It has now been discovered that cyclic amides having unsubstituted hydrogen on the amide nitrogen atom may be utilized as gelation inhibitors or stabilizers for polymerizable epoxy-containing compositions and are capable of extending the pot life or storability of such compositions for extended periods.